1. Field of the Invention
The present invention generally relates to magnetic resonance technology, and specifically to a magnet for a magnetic resonance system, particularly a supported pot magnet.
2. Description of the Prior Art
In a magnetic resonance imaging (MRI) system, a superconducting magnet is used to generate a strong and uniform magnetic field, in which a patient or other subjects are located. A gradient coil and a radio frequency transmitting and receiving coil then exert an influence on the gyromagnetic substance in the subject, thereby exciting a signal capable of being used for the forming of a useful image. Other systems using such coils include a spectroscopic system, a magnetic energy storage system, and a superconducting generator.
Conventionally, the superconducting magnet is placed in a cryostat that has a thermal shield and a vacuum container that isolate the magnet from the external environment during operation. The superconducting magnet further has a coil supporting structure for supporting the coil in a helium vessel and in a cold mass for cooling purposes. The helium vessel is a pressurized vessel located in the vacuum container for thermal insulation, and generally contains liquid nitrogen for cooling the superconducting magnet, so as to maintain the temperature at about 4.2 K, thereby enabling the superconducting effect. However, because liquid helium is a scarce resource, the high cost has an obvious influence on the application of MRI in the field of medical care. Accordingly, reducing the use of liquid helium in superconducting magnets has become the subject of much current research. Compared with a conventional magnet cooling method, the use of a small amount of liquid helium as a circulating coolant in a cooling tube is a quite promising magnet cooling method due to the effective reduction of the usage amount of the liquid helium.
However, in this method, the thermal stability of the superconducting magnet becomes a crucial problem. In clinical scanning application, the gradient coil system transfers excess eddy heat to the magnet, causing thermal quenching of the superconducting coil. Therefore, how to reduce eddies and effectively cool the superconducting coil, are core tasks in implementing the method.
Numerous solutions for using a small amount of liquid helium as the circulating coolant in a cooling tube are described in the patented art. For example, U.S. Pat. Nos. 7,449,889, 7,319,327, 7,319,329, and US Patent Application Publication No. 20090033450 describe methods of reducing eddy heat transfer and improving cooling efficiency in cooling methods where the cooling tube is in contact with the magnet coil. These above patented technologies disclose the use of a non-metallic coil former and a non-metallic suspended structure can greatly reduce eddy heat and improve the cooling efficiency, but there is generally quite a large difference between the thermal contraction coefficients of metallic material and non-metallic material, which causes the superconducting coil to suffer a surface stress resulting from a larger thermal contraction. There are no solutions to this problem provided in the prior art.
Moreover, the problem of fabrication precision is not mentioned in the above patents, but in practice the fabrication precision can largely affect the magnetic field homogeneity, and the surface stress of the magnet coil under thermal contraction and electromagnetic action. Although U.S. Pat. No. 7,319,329 describes a method for reducing heat transfer between the magnetic coil and the coil former, the method is quite complex in implementation, and the assembly precision is difficult to ensure unless there is a fairly advanced fabrication process, making the method difficult to be realized.